sched_4bsd: use the same switch flags as ULE

ULE uses the more specific SWT_REMOTEPREEMPT and SWT_REMOTEWAKEIDLE
switch types, let's do that here as well. SWT_PREEMPT is somewhat
redundant when we also have the SW_PREEMPT flag.

This only has an effect for kernels built with SCHED_STATS.

Reviewed by: kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38183